As a technique of promoting the growth of a so-called higher plant such as a land plant, a photosynthetic dark reaction (Calvin-Benson cycle), which involves the enhancement of the activity of enzyme such as ribulosebisphosphate carboxylase, has previously been reported. Specifically, a technique of enlarging leaves by introducing a related enzyme gene into such a higher plant has been reported (Shigeoka et al., Nature biotechnology, 19, 965-969 (2001)). However, it has been extremely difficult to apply such techniques to various types of higher plants.
Cytochrome c6 is an electron transfer protein in a photosynthetic light reaction, and in general, it exists only in several types of algae (blue-green algae, etc.) It has been known that cytochrome c6 has excellent electron-transferring ability (that is, its oxidation-reduction potential is high) (FIG. 1). Thus, it has been strongly desired that a general-purpose technique of allowing cytochrome c6 to express and function in the chloroplast (more in detail, in the thylakoid space) of various types of higher plants, so as to improve photosynthetic ability, be developed.
By the way, examples of a technique of allowing cytochrome c6 to express in a cell include those described in publications such as F. P. Molina-Heredia et al., Biochem. Biophys. Res. Commun., 243, 302-306 (1998); T. Satoh et al., FEBS lett., 531, 543-547 (2002); R. Gupta et al., Nature, 417, 567-571 (2002); and D. R. Hickey et al., Gene, 105, 73-81 (1991). However, in all of these techniques, host cells have not been those of higher plants, and cells of Escherichia coli, yeast, or blue-green algae, have been used just for simple purposes, such as mass-production of the aforementioned cytochrome c6 or a protein associated therewith, or the function analysis thereof. Hence, such techniques have been included in the gene expression method, which has been commonly carried out by persons skilled in the art.
There have been no reports regarding that cytochrome c6 is allowed to function as an electron carrier in a photosynthetic light reaction in higher plants, namely, regarding that cytochrome c6 is successfully allowed to exist in the thylakoid space of the chloroplast of a higher plant cell. Thus, such a technique has been considered extremely difficult.